desktop/picom: Fixed ninja build.

This was broken by a6ed8b5

Signed-off-by: Matteo Bernardini <ponce@slackbuilds.org>

Signed-off-by: Willy Sudiarto Raharjo <willysr@slackbuilds.org>

desktop/picom/picom.SlackBuild

@@ -82,8 +82,7 @@ find -L . \
 CFLAGS="$SLKCFLAGS" \
 CXXFLAGS="$SLKCFLAGS" \
 meson build . -D build_docs=true -D b_ndebug=true --prefix=/usr
-"${NINJA:=ninja}"
-DESTDIR=$PKG $NINJA -C build install
+DESTDIR=$PKG "${NINJA:=ninja}" -C build install
 
 find $PKG -print0 | xargs -0 file | grep -e "executable" -e "shared object" | grep ELF \
   | cut -f 1 -d : | xargs strip --strip-unneeded 2> /dev/null || true